Cortana, whcih is one of Windows Phone 8.1's most impressive features, and subsequently a huge feature of Windows 10, has leaked early on Android. Available as a user-installable .apk file, from http://suomimobiili.fi Cortana can be installed on most Android devices and Cortana will hapily check the weather, sports scores or run various other searches. The 'Hey Cortana," command is not yet available in this build but will likely be made available when Microsoft is ready to submit an updated version.
Source: Engadget